Currently thermoplastic resins such as polyethylene (abbreviated as “PE” hereinafter), polypropylene (abbreviated as “PP” hereinafter), and the like, absorbent cotton and paper are generally used as a raw material for sanitary goods such as sanitary tampons, napkins, liners and disposable diapers, and the like. Thermoplastic resins such as PE, PP, and the like among them are widely used because of moldability with high flexibility.
However, molded products using such resins as a raw material do not easily disintegrate their shape on contact with water so that they cannot be flushed into a toilet bowl after use. This is because these molded products cause clogging of a toilet piping and a sewage disposal system when they are flushed in a toilet bowl. Therefore, a user could be constrained to experience such inconveniences as disposing the used sanitary goods in a trash being provided in a toilet or taking them home to dispose when away from home because of anxiety and concern with treatment afterward at outside or visiting location. Thus, it is more hygienic and more convenient if the sanitary goods mentioned above, particularly menstrual sanitary products and disposable diapers can be disposed as an effluent after use similarly to toilet paper, and the like.
With such a background, development of a (water disintegrating) material has been strongly desired, which as a raw material for sanitary goods, does not disintegrate with body fluid when used, but disintegrates on contact with a large amount of water when disposed into a toilet bowl, and the like after use.
Increase of environmental load is generally pointed out when a polymer material without biodegradability is disposed or discharged into nature. Therefore, a biodegradable raw material is preferred as a material discharged into a toilet bowl, and the like from a viewpoint of lowering environmental load and its development has been desired.
Furthermore, paper used as sanitary goods conventionally has such advantages as high water absorptivity and capability to be discharged into a toilet bowl because of a natural raw material, but the sense of touch such as texture and appearance are poor because it lacks moldability with high flexibility as the thermoplastic resins. It is therefore not suitable to use as a surface material of the sanitary goods, in which importance with the sense of touch and appearance is stressed. Accordingly, in order to improve the sense of touch and appearance of a raw material for the sanitary goods and the like, moldability is also required for the raw material.
From the above, creation of a raw material satisfying the following three features is desired for a raw material used in the sanitary goods.
1. Good stability and no deterioration in strength against moisture in the air or a small amount of water, but good water disintegratability with easy disintegration of its shape on contact with a large amount of water, particularly contact under a neutral condition.2. Biodegradability without environmental load on the earth.3. Moldability to maintain good sense of touch such as texture and good appearance.
Poly(3-hydroxylactic acid) is disclosed in Patent Document 1 as a method to solve the problems above. In this disclosure, a biodegradable thermoplastic resin is used to solve the problems with biodegradability and moldability, but fails to solve the problems with the water disintegratability because poly(3-hydroxylactic acid) lacks water disintegratability.
Patent Document 2 discloses as an example that improved this problem a method to disintegrate poly(3-hydroxylactic acid) with water under a basic condition. This method hydrolyzes the ester bond under a strong basic condition (equal to pH 12 or higher) to disintegrate with water. However, a reaction rate of the ester bond hydrolysis is too slow to fully disintegrate with water, resulting in inadequate solution of the problems with water disintegratability.
Patent Document 3 also discloses an acrylic polymer with a carboxyl group, which demonstrates water disintegratability under weak basic condition (pH 10) as an example that water disintegratability was improved under basic condition. This acrylic polymer with the carboxyl group demonstrates water resistance under neutral condition and water disintegratability by adding a base to water in a toilet bowl, and the like, to adjust to weakly basic, but lacks biodegradability, so that there is still a problem with environmental load.
Next, as an example of a water disintegrative polymer under neutral condition, Patent Documents 4 and 5 in which water soluble resin such as polyvinyl alcohol or polyethylene glycol, and the like were used are published. However, in these examples, there are such problems in use, as the material absorbs moisture with time to get moist before use, causing stickiness on surface or generation of mildew because a water soluble resin is contained in the material.
Patent Document 6 also discloses a biodegradable resin composition obtained by mixing 20 to 80% by weight of a biodegradable plastic and 80 to 20% by weight of a water soluble thermoplastic resin. This biodegradable resin composition comprises a water soluble thermoplastic resin, which is dissolved in or swollen with water to break or potentially disintegrate a shape of a molded product of the biodegradable resin composition, but cannot provide sufficient water disintegratability because the biodegradable plastic itself used lacks the water disintegratability. It has such problems in use as the material absorbs moisture with time to get moist before use, causing stickiness on surface or generation of mildew because of use of a water soluble thermoplastic resin in the material.
In addition to the above, a method to utilize a chemical bond such as an imide or acetal bond, and the like, which is chemically readily hydrolyzed may be considered as an approach to improve the water disintegratability, but a material satisfying three required features described above has not been thus far found.
For example, polyphenyl azomethine having high heat resistance and demonstrating facile degradability only in acidic aqueous solution or a polymer with a cyclic imine structure demonstrating no degradability in a neutral aqueous solution are known as a polymer having the imine bond (also called as azomethine bond) (Patent Document 7).
Various azomethine polymers are further disclosed in Patent Document 8 and Non-patent Documents 1 and 2. These documents do not particularly refer to degradability with water, but all of the azomethine polymers disclosed do not have biodegradability. A polymer with the acetal bond also includes polyoxymethylene, which is one of typical engineering plastics. This polymer has excellent heat resistance, water resistance and durability, which are not predicted from the unstable chemical structure of the acetal bond that constitutes the polymer and is used as a raw material for a plastic water pipe.
Even in a polymer constituted by a chemical bond with facile chemical hydrolyzability, the water disintegratability is very difficult to predict or discuss. A practical material with both water disintegratability and biodegradability together which does not disintegrate in common use, but disintegrates on contact with a large amount of water is not known.    Patent Document 1: Japanese Patent Laid-open Publication No. S63-302845    Patent Document 2: European Patent No. 0142950    Patent Document 3: Japanese Patent Laid-open Publication No. H7-57230    Patent Document 4: Japanese Patent Laid-open Publication No. H5-29211    Patent Document 5: Japanese Patent Laid-open Publication No. H6-134910    Patent Document 6: Japanese Patent Laid-open Publication No. H6-299077    Patent Document 7: Japanese Patent Laid-open Publication No. 2003-73470    Patent Document 8: WO 2004/003044    Non-patent Document 1: J. Macromol. Sci., Chem., A1 (7), 1161-1249 (1967)    Non-patent Document 2: Chem. Commun., 1522-1524 (2005)